The present invention relates to a multimedia communication terminal for reproducing video data delivered through a bi-directional transmission cable, and more particularly to a monitoring system for monitoring status of the multimedia communication terminal and detecting an abnormal state occurring there in reproducing the video data.
Video data service by way of a bi-directional transmission cable such as CATV has become popular according to diversification of personal or business demands on video information. Especially, technical developments on an interactive television service called VOD (Video On Demand) are active.
FIG. 11 is a block diagram illustrating a CATV system disclosed in a Japanese patent application laid open as a Provisional Publication No. 324784/'92 for realizing the VOD service with simple equipment.
Referring to FIG. 11, there is connected a center system so-called a head end system 11 at an end of a transmission line 12, and each of the other ends of the transmission line 12 is connected to a STB (Set Top Box) 13 provided at each of a plurality of subscribers of the CATV system. The head end system 11, having recording media or a video recorder, supplies multimedia data and takes charge of the supply control. The STB 13 functions as a multimedia communication terminal and is connected to a video display 14 having an interface with the STB. In the CATV system of FIG. 11, there is also provided a special channel selector SCS 15, which is controlled only from the head end system 11 to be tuned to one of transmission channels to be connected to a special program channel of a program selector of the STB 13.
In the CATV system, when a subscriber designates a specified program so as to be delivered at a specified time, the supplier at the head end system 11 remote-controls the tuning circuit of the SCS 15 to be tuned to a transmission channel through which the specified program is delivered, so that the subscriber can enjoy the specified program by operating the program selector of the STB 13 to select the special program channel connected to output of the SCS 15.
This is an example of the VOD system.
There is also proposed and being developed a CATV system called an N-VOD (Near Video On Demand). In an N-VOD system, subscribers previously register programs and time zones they desire to be reserved and the service provider delivers multimedia data of a program at a time zone from the head end system through the transmission line to subscribers who reserved the program at the time zone.
There is also proposed another N-VOD system wherein the head end system delivers multimedia data of the same program through a plurality of transmission channels, with a certain time difference between each other of the plurality of transmission channels, for enabling a subscriber, who intends to enjoy the program, to select one of the plurality of the transmission channels where data of the intended program is to be delivered in his adequate time zone.
In these N-VOD systems, the multimedia data are supplied in a certain time zone and the subscriber should wait for beginning of the desired program when the subscriber turns on the multimedia communication terminal before the beginning, watching his display in vain.
For the purpose, there is proposed an N-VOD system in a Japanese patent application laid open as a Provisional Publication No. 250314/'95, wherein a special sub-channel is selected when a subscriber turns on the multimedia terminal and designated a main channel where his desired program is soon beginning. In the special sub-channel, several short programs are served and at the timing when the desired program begins in the main channel, the multimedia terminal is controlled to be turned over to the main channel, enabling the subscriber to enjoy the N-VOD system with little incongruity.
In these services, namely, the VOD services or the N-VOD services, the head end system takes charge of a supply control and, in the sense, the service provider is responsible for a good service delivered to each of the subscribers, and there may be always a chance wherein an abnormal state occurs in the multimedia communication terminal by an unexpected obstacle, even when all other equipment are working normally.
Accordingly, an effective method has been needed for detecting the abnormal state in the multimedia communication terminal and reporting it to the head end system.
In a Japanese patent application laid open as a Provisional Publication No. 45698/'88, there is proposed a method for detecting an abnormal state from picture data of a monitor ITV, wherein brightness value of pixel data A/D-converted of a moving picture frame is compared with that of a picture frame taken previously by 0.5 to 1 second. When the brightness difference between two picture frames remains within a threshold value for a certain period, it is decided that an abnormal state has occurred in the monitoring object, in a production robot, for example, which should work without cease.
In another Japanese patent application laid open as a Provisional Publication No. 51141/'85, there is proposed a method for detecting runaway of software, wherein is implemented a runaway watch program that periodically interrupts the main program at a low priority of interruption. When the runaway watch program ends normally, a performance confirmation pulse is generated to be counted by a pulse counter. Another counter counts the number of interruptions programmed to occur in a certain interval. Thus, a software runaway is detected, in the proposed method, by comparing the number of the performance confirmation pulses with the number of interruptions, as no performance confirmation pulse is generated when the runaway watch program does not end normally.
However, these prior methods for detecting abnormal states are a little too complicated to be applied widely for detecting abnormal state of the multimedia communication terminal. Moreover, there may be cases where picture brightness variance remains within the threshold value even when the multimedia communication terminal is operating normally, or cases, on the contrary, where some obstacle is seen even when software in the multimedia communication terminal is operating normally.
Furthermore, in the N-VOD systems such as previously described, there may be scheduled interruptions of multimedia data supply, where no picture is reproduced despite no abnormal state of the multimedia communication terminals.